This invention relates generally to the field of silicon purification, and more particularly to a method for improving the efficiency of such a silicon purification process
The production of high purity electronic grade silicon is the critical first step of the entire multi-billion dollar semi-conductor industry. The basic process consists of three steps; conversion of metallurgical grade silicon into a hydrohalosilane such as trichlorosilane, purification of this material by distillation and other means, and decomposition of the material back to silicon.
There are two established ways to produce the hydrohalosilane; a low temperature (300-400 C.) low pressure (1-5 atm) high yield (90%) process using a hydrohalide, such as hydrogen chloride; and a high temperature (400-500 C.) high pressure (30-40 atm) low yield (12-24% depending on catalyst and conditions) process using silicon plus hydrogen to hydrogenate a silicon tetrahalide such as silicon tetrachloride.
The hydrohalide process was invented by Siemens and is used by the majority of the silicon producers; the hydrogenation process was invented by Union Carbide and is used in two facilities as part of their silane process.
The following equations show the desired reactions for the two processes but as noted above reaction 1 has a much higher yield
Si+3HClxe2x86x92SiHCl3+2H2 Hydrohalide processxe2x80x83xe2x80x831
Si+3SiCl4+2H2xe2x86x924SiHCl3 Silicon tetrachloride hydrogenationxe2x80x83xe2x80x832
Purification is normally done by distillation, but reactive distillation is also used, as is adsorption. In most facilities there is extensive recycle and purification of hydrogen. Three facilities produce silicon hydride or silane, two from the hydrohalosilane by disproportionation (Union Carbide Process) and one from silicon tetrafluoride by reduction via aluminum hydride (Ethyl Process).
The decomposition reactors are all rod reactors except for fluid bed reactors operated on silane as part of the Ethyl Process. Fluid bed reactors have significant capital, operating and energy advantages but have proved difficult to implement. The only operating fluid bed units produce a dusty product contaminated with hydrogen that is not widely accepted.
There are two decomposition reactions for hydrohalosilanes; thermal decomposition and hydrogen reduction. (Trichlorosilane is used in the examples but bromine or iodine can be substituted for chlorine, fluorine cannot)
4SiHCl3xe2x86x92Si+3SiCl4+2H2xe2x80x83xe2x80x83(thermal)
SiHCl3+2H2xe2x86x92Si+3HClxe2x80x83xe2x80x83(hydrogen reduction)
All halosilane reactors incorporate both and consequently produce an effluent, which has a range of silicon hydrohalides and tetrahalides and hydrogen halides and hydrogen.
The essence of the process is impure silicon in, pure silicon out plus small impurity streams. To accomplish this there are large recycle streams of hydrogen, silicon and halide containing streams and is important not to produce low value by-products or waste streams.
The key problem is the silicon tetrahalide, which is difficult to convert to silicon, and thus causes a difficult problem in closing the plant silicon and chlorine balances without large waste streams. In the preferred hydrohalide reaction process the silicon tetrahalide is also produced albeit in small quantities (4-5%). Thus there is a net production of silicon tetrahalide as a byproduct. Most plants try to minimize this byproduct production and then convert what they have to fumed silica, which is not as valuable as electronic grade silicon but enables recovery of the hydrogen halide for reuse. The Union Carbide hydrogenation of silicon tetrahalide was invented to overcome this problem but is an expensive and dangerous solution (one accident and two fatalities have been reported to date). Another approach was taken by Wacker-Chemie as is shown in the U.S. Pat. No. 4,454,104 by Griesshammer where silicon tetrachloride is reacted with hydrogen in a reactor parallel to the deposition reactor. The effluent from both reactors are then mixed and compressed to 8 bar and cooled to xe2x88x9260 C. in order to force the hydrogen chloride into solution and allow separation of the hydrogen. No mention is made in this patent of the benefits of controlling the temperature of the effluent or of quenching the reaction.
In a recent patent U.S. Pat. No. 5,910,295 by de Luca a closed loop process is proposed which combines the Union Carbide approach of producing silane by disproportionation with the Siemens approach of producing trichlorosilane from hydrogen chloride at high yield. The solution is to react the excess silicon tetrachloride with hydrogen and oxygen. The overall silicon production reaction is simply the thermal decomposition of trichlorosilane to silicon and silicon tetrachloride. The excess silicon tetrachloride is then oxidized with hydrogen and oxygen which chemically is the same as reacting with water.
4SiHCl3xe2x86x92SiH4+3SiCl4
SiCl4+2H2Oxe2x86x92SiO2+4HCl
Thus 3 moles of fumed silica are produced for every mole of silicon produced. Thus the trichlorosilane production reactors and purification processes must be four times larger than if all the silicon in the trichlorosilane were converted to silicon. The increased capacity of the above process is used to make fumed silica, which is a much less valuable product and does not require the high purification levels that the electronic grade silicon product does. Other silicon production processes make great efforts to promote the hydrogen reduction reaction because it produces more silicon from a mole of trichlorosilane.
SiHCl3+2H2xe2x86x92Si+3HCl
Such efforts typically include running the reactor at higher temperatures (1100 C.) than needed for thermal decomposition (850 C.) and recycling silicon tetrachloride and hydrogen to the reactor until the silicon tetrachloride is consumed. All practical plants also convert some product to fumed silica either as a means of disposing of contaminated material or as a byproduct for sale. For an overall optimum facility one would want to have the flexibility of producing the desired product slate of silicon and amorphous silica depending on market conditions. Such optima depend on the market demand and pricing for the products together with the marginal production cost and equipment capability and can be optimized using linear programming techniques, as is done in oil refining, providing the equipment has some flexibility.
There have been a number of patents for silicon deposition reactors; the key rod reactor patent is the U.S. Pat. No. 3,041,14 by Schering. U.S. Pat. No. 4,092,446 by Padovani describes an optimized system using a fluid bed and extensive recycle of materials. U.S. Pat. Nos. 5,798,137 and 5,810,934 by Lord describe a fluid bed capable of operating with or without recycle on a variety of feedstock. Various fluid bed patents describe methods of operating and of heating. U.S. Pat. No. 5,374,412 by Kim et al. describe use of two feed streams one of which is used to prevent wall deposition which would block the passage of the microwaves used for heating the beads.
All these systems take the effluent from the decomposition reactor as it is cooled down and removed from the reactor and then separate and recycle the components.
The primary deficiency in the prior technology is that it neglects the opportunities in the temperature regime between the deposition temperature which is typically between 750 and 1150 C. and the condensation temperature of the halosilanes in the effluent which are typically below room temperature. The effluent gases are allowed to cool and continue to react through this large temperature range thus producing more of the undesired silicon tetrahalide.
In this range the species in the effluent change composition with temperature and there is always an optimum temperature for recovery of the desired components which is typically 800-1000 C. At this temperature the desired hydrohalosilanes such as trichlorosilane and dichlorosilane are at or near a maximum and thus can be recovered which has great impact on the overall silicon and chlorine balance.
Thus, because of this deficiency the reactors must operate hotter and with greater hydrogen recycle to try and convert the undesired silicon tetrachloride. This in turn results in lower silicon production, more difficult materials problems and greater energy requirements.
A further deficiency is that the opportunities to change the composition of the effluent gases so as to recover more of the valuable hydrohalosilane feedstock are also neglected. The current approach of recycling a reaction byproduct (silicon tetrachloride) to the decomposition reactor results in decreased in production of that product but also results in decreased production of the desired product. The alternate approach, used by Griesshammer, of a parallel reactor reducing silicon tetrachloride with hydrogen converts a small percentage, 10%, of the silicon tetrachloride, requires heating both streams, does not promote silicon production, and significantly lowers the trichlorosilane to silicon tetrachloride ratio in the effluent. [This dilution of the feedstock is desirable for the Griesshammer patent as it improves the solubility of the hydrogen chloride but it increases the cost of the later separation.]
A further deficiency of the present processes is that the reactor does not have much flexibility in consumption of silicon tetrachloride so the byproduct silicon tetrahalide must be used to make fumed silica even if the market demand is not present.
This problem becomes more acute with the use of fluid bed reactors because they are more susceptible to materials problems as the silicon product is in physical contact with the wall, which thus must be at or close to the deposition temperature. This requires hot walls in contrast to the rod reactors, which typically have cooled walls. Furthermore fluid bed reactors do not have the internal heat generation provided by the electrical heating of the rod in rod reactors and so must add heat in some other way. If this heat is added through the walls, the walls must be hotter than the silicon product. U.S. Pat. No. 4,207,360 to Padovani described one solution to this problem, which was to use a graphite lining coated with silicon carbide. Unfortunately this material contaminated the silicon produced with carbon and thus the process failed commercially as the silicon could not sold. Lord points out that the use of silicon oxide is preferred because of its purity and cost and it has some temperature limitations. A further problem is that the materials coming into the reactor can only be preheated to a temperature below their thermal decomposition temperature which is 350-450 C. for most feedstock materials. For high throughput fluid bed reactors putting in the additional heat to bring the temperature up to the desired decomposition temperature of greater than 800 C. is very difficult. Lord suggests use of laser and/or chlorine heating in conjunction with microwaves. Heating the reactor up to the even higher temperatures needed to convert silicon tetrahalides make this problem even worse. Operating at lower temperatures (600-700 C.) as is done in U.S. Pat. No. 4,784,840 requires low deposition rates and results in dusty product contaminated with hydrogen thus requiring post treatment as described in U.S. Pat. No. 5,242,671. A further disadvantage of the prior technology is that the high temperatures used cause formation of silicon dichloride SiCl2 monomer which condenses and polymerizes on the walls of the effluent piping to form explosive solids such as Si2Cl6, Si3Cl8 and Si4Cl8. One approach to resolving this problem is the injection of chlorine or hydrogen chloride in the effluent piping as suggested by Lord. The present technology encourages the reaction of such monomers to form useful products and by keeping the walls of the recovery reactor warm discourages condensation of these species. The selection of optimum recovery temperature can take into account formation of these polymers and thus it may be more optimal to select a lower temperature than would be the case otherwise in order to reduce the operational problems.
The primary object of the invention is to increase the overall efficiency of processes for the production of high purity silicon.
Another object of the invention is to recover the maximum amount of valuable feedstock.
Another object of the invention is to reduce unwanted byproducts.
A further object of the invention is to make it easier to use fluid bed reactors.
Yet another object of the invention is to simplify the retrofit of fluid bed reactors for rod reactors.
Still yet another object of the invention is to use the cheaper hydrohalide reaction for halosilane production.
Yet another further object is to improve the quality of product produced by fluid bed reactors
In accordance with a preferred embodiment of the present invention, a method for improving the efficiency of a silicon purification process comprises the steps of controlling the temperature and composition of the effluent to an optimum feedstock recovery composition and temperature, rapidly quenching the effluent at or near the recovery composition, separating the gases from the liquids, sending the gases to conventional hydrogen recovery and recycle facilities, separating the hydrohalosilanes from silicon tetrahalide, returning the hydrohalosilanes to the inlet of the deposition reactor and using at least some of the silicon tetrahalide to control the composition and temperature of the effluent and separately heating and injecting the hydrogen and silicon tetrahalide feed streams to the deposition reactor to a temperature above 350 C.
In the proposed invention all or some of the silicon tetrachloride bypasses the deposition reactor to the recovery reactor while all the hydrogen goes through the deposition reactor thus improving the yield of the hydrogen reduction reaction. This results in increased yield in the deposition reactor, a reduced flow to the deposition reactor and an increased recovery of feedstock. Combining the two improvements of controlling the temperature and the composition of the effluent also results in equipment and energy savings as the added silicon tetrachloride can be used to cool the effluent and hence does not itself require to be heated to the recovery reactor temperature.
Other objects and advantages of the present invention will become apparent from the following descriptions, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed.